Signs of Summer 6: Cavity Nesting Team, Year #4!

Photo by D. Sillman

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Over the past four years I have written a number of articles about our Cavity Nesting Team study up at Harrison Hills Park in northern Allegheny County. This year we have twenty-nine nesting boxes scattered across the park and the Team started our 2018 monitoring on March 31. We have built up a steady accumulation of observations and hypotheses during the years of our study. Our findings from 2015 helped us better understand the optimal location variables for our nesting boxes (and our relocated 2016 boxes were almost all utilized for nests!). Our 2016 data helped us design two experiments for 2017 in which we tried to regulate house wren nesting in our boxes (house wrens are very destructive to nesting blue birds and tree swallows). The results of our experiments in 2017 were not very successful but we re-tuned some of our 2017 ideas for the 2018 season and may have finally achieved a sustainable equilibrium between blue birds, tree swallows and house wrens.

This year’s Cavity Nesting Team consists of ten volunteers: Deborah and I and Sharon Svitek take turns monitoring the boxes in and around the “High Meadow.” Patrick and Mardelle Kopnicky and Dave and Kathy Brooke check the boxes around the “Bat House Meadow.” Chris Urik and Kristi Cihil take turns monitoring the boxes at the park entrance and up in the fields near the Environmental Learning Center, and Paul Dudek checks the boxes around the pond and soccer fields in the southern end of the park. We also had a great group of student volunteers from the Harvest Baptist Church who helped us out during our May monitoring.  Data from our observations are uploaded to an on-line Google spreadsheet, and each week Deborah compiles and distributes the growing data tables to each member of the team. Also, each year Chris Urik has made new GPS maps of the park showing the precise location of each nesting box.

Photo by D. Brooke

As I have talked about before, native cavity nesting bird species (eastern bluebirds, tree swallows, house wrens, Carolina wrens, titmice, chickadees, nuthatches, etc.) naturally use tree holes for their nesting sites. These holes are most often found in older, often dead or dying trees, and they are typically abandoned cavities that have been chiseled out by woodpeckers. The lack of the these older trees in most forests has led to a “housing crisis for these cavity nesting species. Nest boxes, of course, are artificial substitutes for these natural tree holes.

So what have we seen so far in 2018?

Blue bird eggs. Photo by C. Urik

We saw our first blue bird nest on April 19 in a nesting box near the park’s Environmental Learning Center, and the first eggs were seen a week and a half later (April 28). Two weeks later (May 14) these eggs had hatched into nestlings and by then a large number of other boxes throughout the park were being utilized by nesting blue birds. In the first six weeks of our study eleven boxes had blue bird nests and thirty-three eggs were counted. Also during this initial period of the season, two boxes had tree swallow nests started (no eggs yet) and four boxes had house wren nests (also no eggs yet).

In the next three weeks two more boxes (13 total) had blue bird nests (43 eggs total with 28 nestlings and 24 fledges), one more tree swallow nest was built (3 total boxes with 15 eggs and 7 nestlings), and four more boxes had house wren nests (6 total boxes with one observed egg). A number of these house wren nests were “dummy” nests built by male house wrens as part of their mating displays. We diligently removed these practice nests in an attempt to distract the wrens from actual nesting. We also observed in this time frame one chickadee nest (0 eggs).

In the next four weeks blue bids added four more nests in boxes (17 total (3 more than in 2017!)). There were 63 blue bird eggs counted and 33 nestlings and 25 fledges observed. The “spring/early summer” nesting period of the blue birds occurred right on schedule! Some new blue bird nests were observed on July 1 indicating the start of the blue birds’ second, “late summer” reproductive phase.

Tree swallow nestlings. Photo by D. Brooke

Four nesting boxes had tree swallow nests and 20 eggs were observed. These eggs were primarily laid during the “summer” time period after the initial blue bird reproductive cycle but before their later “late summer” second reproductive event. This tree swallow egg total was very comparable to our 2015 observations and was a significant rebound from the very low tree swallow eggs production of 2016 and 2017. We had speculated that the dry summers of 2016 and 2017 had inhibited the emergence of the aquatic insects upon which the swallows rely to feed their nestlings. The very wet summer of 2018 must have generated a very rich population of these important food items. Further, all twenty tree swallow eggs resulted in viable nestlings, and all twenty nestlings successfully fledged.

Nine nesting boxes had house wren nests and 13 eggs were observed. One of the wren nests was the chickadee nest noted in the previous time period. The wrens usurped the nest from the chickadees and laid their eggs in the formed nest (it is not known if the chickadees had laid eggs or if chickadee nestlings had hatched in the nest, but if they had, they were destroyed by the wrens). No fledges from any of the house wren nests, though, have yet been observed. In past years, the house wrens nested and reproduced concurrent with the early blue bird nesting cycle (“late spring/early summer”).This year’s wren reproduction, though, is later in the season (corresponding more to the middle summer reproduction cycle of the tree swallows). Possibly this delay was due to both the Team’s interference with the wrens’ “dummy” display nests and also our moving a number of nesting boxes away from the park maintenance area (an hypothesized habitat refuge for the wrens). So far, the house wren reproduction efforts have not been successful. No fledges have been observed from the 13 eggs!

House sparrow nestlings. Photo by P. Kopnicky

For the first time in our study we have observed house sparrows (“English sparrows”) in one of our nesting boxes. These alien, invasive birds are very destructive (they invade established blue bird and tree swallow nests and destroy eggs and kill both nestlings and adults). The location of the park far from human habitations has insulated us from these birds, but here in the fourth year of our study they have finally shown up. Four eggs developed into four fledglings in one of the boxes near the Environmental Learning Center. It is possible that this house sparrow nest was built on a blue bird nest and possibly even on top of the dead bodies of some blue bird nestlings or adults. Since house sparrows are not native species, we were well in our legal rights to dispose of their eggs and even their nestlings, but none of us had the heart to do so. We hope that the fledged house sparrows will fly off to a habitat more suited to their needs (there is a McDonald’s and a Burger King not all that far away in Natrona Heights). We will, though, keep an eye out for them here in the park!

We still have two months left in our nesting box season. Our monitoring will run through Labor Day!  It is likely, barring some calamity, that we will exceed last year’s record numbers of blue bird nests/eggs/fledges. We have also had a very satisfying reproductive year for our tree swallows! If we can keep the house wrens under control and prevent an invasion of house sparrows, the cavity nesting bird community of Harrison Hills Park looks to be robust and healthy!